Moisture sensors

ABSTRACT

A moisture sensor for a hygrometer and a method of manufacturing such a sensor. The method comprises flattening part of a length of aluminium wire between two elements at least one of which has a smooth flat surface, to form an aluminium strip having at least one smooth flat surface, causing oxidation of at least the smooth flat surface and providing a layer of a second metal over at least part of the oxidized surface, the second metal being preferably a noble metal and securely anchored to the layer by a baking process. After baking the sensor may be boiled in a solution capable of loading the oxide layer with heavy metal ions.

lieu MAlLABLE C Y July 15, 1975 1 MOISTURE SENSORS [75] Inventor: Tibor Dudas, Leeds, England [73] Assignee: Molecular Controls Limited, Leeds,

England [22] Filed: May 24, 1972 [21 Appl. N0.: 256,532

{30] Foreign Application Priority Data May 24, 1971 United Kingdom 16643/71 [52] US. Cl. 317/246; 29/2541; 73/3365 [51] Int. Cl 01g 7/00 [58] Field of Search 317/246, 261; 324/61 P; 73/3365, 335, 204; 204/42; 29/2542, 25.41, 595, DlG. 2

[56] References Cited UNITED STATES PATENTS 1.068.413 7/1913 Chubb 204/58 2.143369 1/1939 Dubilier..... 317/261 3.075.385 l/l963 Stover r 73/335 3.083.573 4/1963 Show 73/336.5 3.523.244 8/1970 Goodman 73/3365 3.683.243 8/1972 Rockliff 317/246 OTHER PUBLICATIONS Handbook of Chemistry and Physics, p. 1621; 38 edition of 1956-1957.

Primary ExaminerS. Clement Swisher Assistant E.\'aminerDenis E. Corr Attorney, Agent, or FirmStevens, Davis, Miller & Mosher [57] ABSTRACT A moisture sensor for a hygrometer and a method of manufacturing such a sensor. The method comprises flattening part of a length of aluminium wire between two elements at least one of which has a smooth flat surface. to form an aluminium strip having at least one smooth flat surface, causing oxidation of at least the smooth flat surface and providing a layer of a second metal over at least part of the oxidized surface. the second metal being preferably a noble metal and securely anchored to the layer by a baking process, After baking the sensor may be boiled in a solution capable of loading the oxide layer with heavy metal ions.

7 Claims, 7 Drawing Figures MOISTURE SENSORS This invention relates to moisture sensors for use in electrical hygrometers, and to a method of manufacturing such sensors.

Two basic forms of moisture sensors are in common use. The first of these comprises an aluminium wire of some 3 to 4 inches in length, surface treated to have an aluminium oxide layer and then coated with a conducting metal layer. The aluminium and the metal layer form a pair of electrodes insulated from each other by the aluminium oxide, and thus forming a capacitance. As the aluminium oxide is hygroscopic it will adsorb moisture from the surrounding atmosphere, the moisture adsorbed being directly related to the humidity. Different moisture adsorption will of course give different capacitance values, and if the electrodes are connected into a suitable circuit, a meter in this circuit can give a reading indicative of humidity. Sensors of this type have the disadvantage that it is difficult to attain a uniform thickness of oxide layer and, to prevent peaks of aluminium wire from contacting the outer metal layer the oxide layer needs to be resonably thick. This thick layer will naturally adsorb more water than would a thin layer, and will correspondingly take longer to dry out when the humidity drops. There is thus a slow response to humidity changes. Furthermore, it is found that as the layer becomes dryer the sensor tends to become impedance as well as capacitance responsive. As a consequence of these problems it has been found necessary to utilize three or four different sensors to cover the full humidity range.

These disadvantages have been partially overcome by the second basic form of sensor, which comprises two aluminium wires loosely twisted together, each wire having a thin oxide coating on the surface. In this case the two wires act as the electrodes of a capacitor and water adsorbed between the wires changes the capacitance in accordance with the humidity. Although electrically more stable and reliable, this arrangement, in which the wires are again of about 4 inches in length is mechanically unreliable, in that vibration, shock and thermal expansion can all affect the calibration of the sensor. 7

The object of the present invention is to provide a moisture sensor which avoids, at least in part, the difficulties associated with known sensors.

According to the present invention a method of manufacturing a moisture sensor comprises flattening part of a length of aluminium wire between two elements at least one of which has a smooth flat surface, to form an aluminium strip having at least one smooth flat surface, causing oxidation of at least said smooth flat surface and providing a layer of a second metal over at least part of said oxidized surface.

The invention also includes a moisture sensor comprising a strip of aluminium having at least one smooth, flat surface, a layer of aluminium oxide on said surface and a layer of a second metal over part of the oxide layer.

The aluminium and the layer of second metal form the electrodes of a condensor, the capacitance of which will change according to the moisture adsorbed by the aluminium oxide.

It has been found that a sensor of this form is highly advantageous. It may be quite short, i.e., about half an inch in length, and this, together with its flattened shape gives excellent mechanical and thermal stability, and avoids any strain on the oxide layer. The accurate flattening of one surface means that the oxide layer can be very much thinner than in previous sensors, so improving the speed of response in both wetter and dryer conditions. It is found that the sensor will give good response over the full possible range of humidity.

Preferably, the smooth, flat surface of said one of said elements is optically flat, and the wire may be flattened between polished stainless steel or armour plate glass plates. Forming the layer of aluminium oxide is conveniently done by conventional anodizing techniques. The second metal is preferably a noble metal, desirably silver, and preferably covers only a small area, particularly less than 25%, of the oxidized flat surface. The surface area of this metal need only be of the order of 2 to 3 sq. mm., compared to areas of l30 sq. mm. in prior sensors, and this greatly reduced area has the advantage that a much smaller volume of water vapor is required to register full humidity change. The second metal may conveniently be deposited by painting or silk screen printing the metal in colloidal form, and when silver is used preferably colloidal silver diluted with amyl acetate is painted onto the surface. After deposition of the second metal the sensor is preferably baked for from 4 to 6 minutes at a temperature of from to C, which is found to improve the bond of the silver onto the sensor and also finely crack the silver to facilitate penetration of water vapour to the underlying oxide.

Preferably the oxide layer is loaded with a heavy ionic substance, and this may for example be effected by boiling the sensor, after deposition of the second metal layer, for from 25 to 35 seconds in a solution of sodium tungstate. While this process is somewhat detrimental to the sensitivity of the sensors it has the advantage that it minimizes the effect of ion loss from the oxide layer and so reduces the effects of ageing on the sensor.

As a more detailed example of the invention, a presently preferred method of making a sensor will now be described, with reference to the accompanying drawing in which the seven figures all illustrate the sensor at different stages of manufacture.

A length of aluminium wire, preferably of 16 or 18 swg., is cut into pieces of the desired length, preferably about three-fourths of an inch, and each piece I of the wire (FIG. 1) has about a-one-half inch length thereof flattened between two plates 2 and 3 as shown in FIG. 2. The flattening is carried out by applying pressure to the plates 2 and 3, and at least the plate 2 is formed with a smooth, flat surface. A flat strip (FIG. 3) of aluminium is thus formed and the surface 4 of the strip has a surface which corresponds in smoothness and flatness to the surface of the plate 2 and is preferably optically flat. This side of the strip is then polished with a fine grade abrasive agent after which the strip is cleaned by boiling in distilled water and wiping with acetone to remove all polish residue and any grease on the surface of the aluminium. Other cleaning methods, for example, ultrasonics, may alternatively be used.

The cleaned strip is then anodized by dipping in a solution of sodium hydroxide or potassium hydroxide for about 45 seconds. A dipping time of much less than 45 seconds may result in an oxide layer of insufficient thickness, whereas much more than 45 seconds immersion seems to dull the performance of the resultant sensor. This anodizing process gives a soft surface layer of aluminium oxide on the strip, and the optimum anodization time of 45 seconds, as compared to some two minutes in previous processes, indicates that a much thinnner oxide layer has been formed. Because the surface 4 of the strip is smooth and flat, however, it is found that this thin layer of oxide is continuous and provides a good insulating dielectric layer. The substantial elimination of peaks and depressions on the aluminium surface means that weak spots in the dielectric oxide layer are reduced to a minimum, and short circuits, even in high humidity conditions, are substantially avoided.

The anodized strip is then rinsed in clean cold water to remove any excess of the anodizing solution, and the strip is then boiled in distilled or soft water for two minutes. This process removes any last traces ofthe sodium hydroxide solution and facilitates proper oxidation of the surface layer of the strip.

The next step, as shown in FIG. 4, is to coat the round end 5 of the aluminium strip with an electrically conductive cement and to mount this end in one end of a tube 6 of electrically conductive material such as brass or copper. The combination of the blade and tube is then baked for about 5 minutes at a temperature of 125 to 150C. to set the cement.

Part of the oxide side 4 of the blade is then covered with a thin but continuous film 7 (FIG. 5) of electrical insulating material, which may preferably be an epoxy resin such as that sold under the trade name *Devcon. One end of a short length 8 of wire, for example tinned wire of 22 swg., is then formed into a loop and placed on the insulating film 7 and is bonded thereto by a blob 9 of electrically conductive cement (FIG. 6). Drying of the cement is then effected at about 100 to 125C. for a period of from 5 to 8 minutes.

- After drying this cement a thin line 10 of the second metal is painted onto the oxide layer to extend onto the insulating film 7 and the blob 9 of conductive cement as shown in FIG. 7. For this purpose a colloidal silver is used, (for example Acheson type DAG 915 is suitable) and this is preferably diluted with two parts of amyl acetate to one part of the colloidal silver as it is found that a better adhesion between the silver and the oxide surface is obtained in comparison to the bond obtained if the colloidal silver is used alone. After painting the silver onto the sensor this is then baked for about 5 minutes at a temperature of about 150C. This has the effect of chemically bonding the silver to the oxide layer and it also cracks the surface of the silver so facilitating penetration of water vapor to the oxide layer.

The sensor is then boiled for about 30 seconds in a five per cent solution of sodium tungstate whereupon the oxide layer becomes loaded with the heavy ionic substance. This has the effect of reducing ion loss from the layer in subsequent use and so retards the gradual deterioration in performance of the sensor.

The sensor is then mounted onto a suitable base fitted with a coaxial coupling, one side of which is connected to the tube 6 and the other side of which is connected to the wire 8. After such mounting the coupling is connected across an alternating current supply which is preferably of about 8 volts at about 1.2 amps. The object of this is to burn out any weak spots on the oxide layer between the aluminium and silver electrodes, and any sensors which break down under this process are rejected. Similarly sensors which spark excessively during this process are also eliminated as they may prove susceptible to breakdown in high humidity conditions. However, it has been found that sensors which satisfactorily withstand this process are sufficiently durable to withstand immersion into such diverse materials as coffee, oil, kerosene, solvents and boiling water without damage. The finished sensors are then desirably stored for from 6 to 8 weeks before use, during which time they are periodically subjected to the alternating current test set forth above. I

It is found that sensors manufactured in accordance with the above described process function equally well in humidity environments ranging from C. to +80C. dew points. The sensors have a fast response to initial changes in humidity, and a rapid recovery time from wet to-dry condition. They are also capable of discriminating between small humidity changes, of the order of l to 2 ppm. The sensors also have a high degree of operational reliability as they are of excellent mechanical and thermal stability and are able to withstand exposure to saturation conditions without subsequent breakdown or malfunction. Furthermore, due to the immersion in the sodium tungstate solution the rate of ion loss from the sensors is reduced and thus the effects of ageing on the sensors are mitigated.

It will be appreciated that the foregoing description is of a preferred process only and that various changes in thematerials and operating conditions in various of the manufacturing steps may be varied. In particular the anodizing may be carried out in two stages, i.e., initial treatment with oxalic acid giving a harder, more brittle layer and then a treatment with sodium hydroxide or potassium hydroxide to give a softer, more absorbent layer." I

1 claim: I i

l. A method of manufacturing a moisture sensor for a hygrometer, the method comprising flattening part of a length of aluminium wire between two elements, at least one of which has a smooth flat surface, to form an aluminium strip having at least one smooth flat surface, oxidizing at least said smooth flat surface in an alkaline solution and providing a layer of a second metal over at least part of the oxidized surface, wherein the alkaline solution is selected from the group consisting of sodium hydroxide solution and potassium hydroxide solution.

2. A method of manufacturing a moisture sensor for a hygrometer, the method comprising flattening part of a length of aluminium wire between two elements, at least one of which has a smooth flat surface, to form an aluminium strip having at least one smooth flat surface, oxidizing at least said smooth flat surface in an alkaline solution and providing a layer of a second metal over at least part of the oxidized surface, in which the second metal is silver and covers less than 25% of the area of the oxidized flat surface, the silver being applied by painting colloidal silver diluted with amyl acetate.

3. A method of manufacturing a moisture sensor for a hygrometer, the method comprising flattening part of a length of aluminium wire between two elements, at least one of which has a smooth flat surface, to form an aluminium strip having at least one smooth flat surface, oxidizing at least said smooth flat surface in an alkaline solution and providing a layer of a second metal over at least part of the oxidized surface, in which after provision of the second metal layer the sensor is baked for from 4 to 6 minutes at a temperature of from 140 to 4. A method of manufacturing a moisture sensor for a hygrometer, the method comprising flattening part of a length of aluminium wire between two elements, at least one of which has a smooth flat surface, to form an aluminium strip having at least one smooth flat surface, oxidizing at least said smooth flat surface in an alkaline solution and providing a layer of a second metal over at least part of the oxidized surface, in which after provision of the second metal layer the sensor is boiled for from 25 to 35 seconds in a solution of sodium tungstate.

5. A method of manufacturing a moisture sensor for a hygrometer, the method comprising flattening part of a length of aluminium wire between two elements, at least one of which has a smooth flat surface, to form an aluminium strip having at least one smooth flat surface,.

oxidizing at least said smooth flat surface in an alkaline 7. A moisture sensor comprising a strip of aluminium having at least one smooth, flat surface, a layer of aluminium oxide on said surface, said layer being formed by treating said surface in an alkaline solution, and a layer of a second metal over part of the oxide layer, wherein the alkaline solution is selected from the group consisting of sodium hydroxide solution and potassium hydroxide solution. 

1. A method of manufacturing a moisture sensor for a hygrometer, the method comprising flattening part of a length of aluminium wire between two elements, at least one of which has a smooth flat surface, to form an aluminium strip having at least one smooth flat surface, oxidizing at least said smooth flat surface in an alkaline solution and providing a layer of a second metal over at least part of the oxidized surface, wherein the alkaline solution is selected from the group consisting of sodium hydroxide solution and potassium hydroxide solution.
 2. A method of manufacturing a moisture sensor for a hygrometer, the method comprising flattening part of a length of aluminium wire between two elements, at least one of which has a smooth flat surface, to form an aluminium strip having at least one smooth flat surface, oxidizing at least said smooth flat surface in an alkaline solution and providing a layer of a second metal over at least part of the oxidized surface, in which the second metal is silver and covers less than 25% of the area of the oxidized flat surface, the silver being applied by painting colloidal silver diluted with amyl acetate.
 3. A method of manufacturing a moisture sensor for a hygrometer, the method comprising flattening part of a length of aluminium wire between two elements, at least one of which has a smooth flat surface, to form an aluminium strip having at least one smooth flat surface, oxidizing at least said smooth flat surface in an alkaline solution and providing a layer of a second metal over at least part of the oxidized surface, in which after provision of the second metal layer the sensor is baked for from 4 to 6 minutes at a temperature of from 140* to 160*C.
 4. A method of manufacturing a moisture sensor for a hygrometer, the method comprising flattening part of a length of aluminium wire between two elements, at least one of which has a smooth flat surface, to form an aluminium strip having at least one smooth flat surface, oxidizing at least said smooth flat surface in an alkaline solution aNd providing a layer of a second metal over at least part of the oxidized surface, in which after provision of the second metal layer the sensor is boiled for from 25 to 35 seconds in a solution of sodium tungstate.
 5. A method of manufacturing a moisture sensor for a hygrometer, the method comprising flattening part of a length of aluminium wire between two elements, at least one of which has a smooth flat surface, to form an aluminium strip having at least one smooth flat surface, oxidizing at least said smooth flat surface in an alkaline solution and providing a layer of a second metal over at least part of the oxidized surface, in which the coated and finished aluminium strip is mounted on a holder fitted with a coaxial coupling, the two parts of which are electrically connected respectively to the aluminium and to the second metal.
 6. A method according to claim 5 in which after mounting the coaxial coupling is connected across an A.C. supply of from 6 to 10 volts and 1.0 to 1.4 amp. for a short period.
 7. A MOISTURE SENSOR COMPRISING A STRIP OF ALUMINIUM HAVING AT LEAST ONE SMOOTH, FLAT SURFACE, A LAYER OF ALUMINIUM OXIDE ON SAID SURFACE, SAID LAYER BEING FORMED BY TREATING SAID SURFACE IN AN ALKALINE SOLUTION, AND A LAYER OF A SECOND METAL OVER PART OF THE OXIDE LAYER, WHEREIN THE ALKALINE SOLUTION IS SELECTED FROM THE GROUP CONSISTING OF SODIUM HYDROXIDE SOLUTION AND POTASSIUM HYDROXIDE SOLUTION. 